Liu Chao-Kang, Ku Yun-Cheng, Kuo Mao-Kuen, Liaw Jiunn-Woei
Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan.
Department of Mechanical Engineering, Chang Gung University, Taipei, Taiwan.
Sci Rep. 2024 Nov 4;14(1):26714. doi: 10.1038/s41598-024-77413-7.
The optomechanical motion of a gold nanoparticle (GNP) dimer-a pair of optically bound GNPs-in fluid, manipulated by a Bessel beam, is theoretically studied using the multiple multipole (MMP) method. Since a Bessel beam possesses orbital angular momentum (OAM) and spin angular momentum (SAM) simultaneously, complicated rigid-body motions of the dimer can be induced. The mechanism involves the equilibrium between the optical force with the reactive drag force exerted by the fluid. Our results demonstrate that the dimer rotates around its center of mass (COM), while the COM performs an orbital revolution around the optical axis. Additionally, each individual GNP undergoes spinning. The directions of the GNPs' spin and the orbital revolution of COM depend on the handedness and the order (topological charge) of Bessel beam, respectively. Nevertheless, the rotation direction of the dimer depends on the size of GNP. In the case of a smaller dimer, the direction of dimer's rotation with respect to the COM is consistent with the handedness of the light. Conversely, a larger dimer performs a reverse rotation, accompanied by a precession during the orbital revolution. There are multiple turning points in the radius of the GNP for the alternating rotation of the dimer caused by positive or negative optical torque. Our finding may provide an insight to the optomechanical manipulation of optical vortexes on the motions of GNP clusters.
利用多重多极子（MMP）方法，对由贝塞尔光束操控的流体中一对光学束缚的金纳米粒子（GNP）二聚体的光机械运动进行了理论研究。由于贝塞尔光束同时具有轨道角动量（OAM）和自旋角动量（SAM），因此可诱导二聚体产生复杂的刚体运动。其机制涉及光学力与流体施加的反应性拖曳力之间的平衡。我们的结果表明，二聚体绕其质心（COM）旋转，而质心则绕光轴进行轨道公转。此外，每个单独的金纳米粒子都会自旋。金纳米粒子的自旋方向和质心的轨道公转方向分别取决于贝塞尔光束的手性和阶数（拓扑电荷）。然而，二聚体的旋转方向取决于金纳米粒子的大小。对于较小的二聚体，二聚体相对于质心的旋转方向与光的手性一致。相反，较大的二聚体会进行反向旋转，并在轨道公转过程中伴随进动。由于正或负光学扭矩导致二聚体交替旋转，金纳米粒子半径存在多个转折点。我们的发现可能为光学涡旋对金纳米粒子团簇运动的光机械操纵提供见解。
